1. Field of the Invention
The present invention relates to camera systems, and more particularly to a multiband camera system having multiple image sensors and an internal alignment assembly.
2. Description of the Related Art
For many military and civilian applications, such as industrial machine vision, laser-beam profiling, military imaging, etc., there is a need for simultaneous capture of high resolution images in multiple frequency bands, such as, for example, shortwave infrared and visible frequencies, thermal and visible frequencies, etc. However multiple band cameras are difficult to produce.
One dual band unit camera is capable of imaging visible and shortwave infrared (SWIR) wavelengths. The key element in this dual band unit camera is a backside-illuminated 320×240-pixel InGaAs photodiode array, with a thinned InP substrate. Visible light is typically absorbed by the InP substrate because of its 920 nm cutoff. However, a thinned InP substrate can let more visible light reach the InGaAs absorption layer. The thinned InP substrate is just thick enough to passivate the InGaAs surface and provide a contact layer for other camera parts. However, such processing is expensive and inefficient. Moreover, the detection band in such cameras is limited by the nature of the sensor material. For example, InGaAs is only sensitive to 400-1700 nm band.
Another known multiband imaging technique, generally represented by FIG. 1, utilizes a multiband camera system 90 including: an optical front end system 29; a first sensor 137, which is optimized for a first frequency band; a second sensor 139, which is optimized for a second frequency band; and a splitter device 35. The splitter device 35 directs some rays from an external object to the first sensor 137 to form a first object image, and directs other rays from the external object to the second sensor 139 to form a second object image. Thus, the multiband camera system 90 images an external object on both the first sensor 137 and the second sensor 139 to obtain images in different frequency bands. Although this arrangement produces images in multiple frequency bands, operating conditions such as vibrations, thermal conditions etc. may cause relative movement between the multiple image sensors. This relative movement results in misalignment when the images are overlaid.
Disclosed embodiments of this application address these drawbacks of known multiband camera systems by providing a multiband camera system having separate sensors for generating images in different frequency bands and an internal alignment assembly that enables accurate image registration even when the sensors are subject to relative motion. In one embodiment, image readout from the multiple image sensors is adjusted based on an internal alignment assembly to automatically align the multiple images while minimizing processing overhead.